This invention relates to a water-based treatment composition for application to metal surfaces. The treatment composition can be used to form, on the metal surface, resin coatings that exhibit an excellent corrosion resistance in flat areas, an excellent corrosion resistance in damaged areas, and an excellent paint adherence. This invention also relates to surface-treated metal articles of manufacture, particularly metal sheets, on which such a resin coating has been formed.
Zinc- and zinc alloy-plated steel sheet is in wide use, for example, for household electrical appliances and building materials. By itself, zinc-surfaced steel sheet of this type has an inadequate corrosion resistance and paint adherence to its zinc-rich surface and for this reason is typically subjected to a chromate conversion treatment or phosphate conversion treatment prior to being subjected to mechanical forming operations (e.g., press working or bending) and/or being painted. However, a substantial amount of zinc-surfaced sheet is used without being painted.
The type of surface-treated zinc-surfaced steel sheet known as xe2x80x9cchromate conversion-coatedxe2x80x9d has frequently been used in these unpainted applications. However, chromate conversion-coated surfaces suffer from color variations caused by differences in the chromate coating weight and also retain. fingerprint impressions that may be made during forming operations and assembly. Fingerprint-resistant zinc-surfaced steel sheet has been used in order to overcome these problems. This fingerprint-resistant zinc-surfaced steel sheet carries an organic coating formed over the chromate coating. With the goal of preventing fingerprint uptake, this fingerprint-resistant sheet is produced by laying down an organic resin layer with a thickness around 1 micrometre (hereinafter usually abbreviated as xe2x80x9cxcexcmxe2x80x9d) after the chromate treatment has been executed on the surface of the zinciferous-plated steel sheet. In addition to fingerprint resistance, the coating on fingerprint-resistant steel sheet must exhibit a variety of properties, such as corrosion resistance, solvent resistance, paint adherence, and damage resistance.
Among these various properties, the ability to resist damage has been in strong demand in recent years. This property is in demand in order to resist the damage to molding surfaces that can be produced when the vibrations generated during the transport of molded articles cause the moldings to rub against one another or to rub against their containers (e.g., cardboard). This makes impairments in quality inevitable, since these damaged areas exhibit a poorer corrosion resistance than ordinary organic-coated zinc-surfaced steel sheet.
In response to this circumstance, various technologies related to surface-treated zinc-surfaced steel sheet have appeared that take into account damage resistance in addition to corrosion resistance and paint adherence. These technologies can be exemplified by the methods described in Japanese Laid Open (Kokai or Unexamined) Patent Application Number Hei 3-17189 (17,189/1991), Japanese Published (Kokoku or Examined) Patent Application Number Hei 6-104799 (104,799/1994), and Japanese Laid Open (Kokai or Unexamined) Patent Application Number Hei 6-292859 (292,859/1994).
Japanese Laid Open (Kokai or Unexamined) Patent Application Number Hei 3-17189 discloses a method that relates to resin coatings comprising fluororesin powder and silica powder blended into urethane-modified polyolefin resin. The characteristic feature of this method resides in the use of the fluororesin powder to protect damaged areas. However, this method requires the use of surfactant in order to generate a uniform dispersion of the fluororesin powder in a water-based solution. The use of this surfactant results in an overall lower level of corrosion resistance and thereby prevents the development of a satisfactory corrosion resistance.
Japanese Published (Kokoku or Examined) Patent Application Number Hei 6-104799 discloses a method that relates to coatings that contain polyester resin, cross-linker, and polyethylene wax with an average molecular weight of 2,000 to 8,000. Due to the use in this method of polyester resin as the base resin, the resulting coating itself has an inadequate resistance to hydrolysis, which again prevents the development of a satisfactory corrosion resistance.
Japanese Laid Open (Kokai or Unexamined) Patent Application Number Hei 6-292859 discloses a method that relates to a coating afforded by the addition of spherical oxidized polyethylene wax powder and chain (coagulated network structure) colloidal silica to resin itself afforded by the addition of ambient temperature-cross-linking epoxy resin to active hydrogen-functional urethane resin. Colloidal silica, upon its adhesion to solid surfaces, functions to raise the friction coefficientxe2x80x94a property known as friction enhancement. In the case of use of chain (coagulated network structure) colloidal silica as in the method under discussion, the damage resistance is impaired by the structure of the colloidal silica itself. This method therefore also uses spherical polyethylene wax, but since this method uses a drying temperature lower than 100xc2x0 C. the polyethylene wax ends up buried in the resin coating. The lubricity therefore remains inadequate and a satisfactory corrosion resistance at damaged areas cannot be developed.
Thus, as described above, it has not been possible using the heretofore disclosed technologies to produce surface-treated zinc-surfaced steel sheet that exhibits an excellent corrosion resistance and paint adherence and also an excellent corrosion resistance in damaged areas.
Japanese Laid Open (Kokai or Unexamined) Patent Application Number Hei 5-118550 (118,550/1993), Granted Japanese Patent 2,719,571, and Granted Japanese Patent 2,617,838 describe (water-based) lubricating paints that contain urethane resin and/or epoxy resin, silica or silica powder, and wax (including polyethylene wax) and that are used to form resin coatings on cold-rolled steel sheet, zinciferous-plated steel sheet, or aluminiferous metal sheet. The invention described hereinbelow differs from these inventions in having different compositional requirements and different advantageous effects.
This invention is directed to solving the problems associated with the prior art as described above. An object of this invention is to provide a water-based agent, for treating metal surfaces, that can be used to form an organic resin coating that exhibits an excellent corrosion resistance, excellent paint adherence, and in particular an excellent resistance to corrosion in damaged regions. An additional object of this invention is to provide surface-treated metal sheet as afforded by the use of the water-based surface treatment composition according to this invention.
It has been found that these problems can be solved by the use of a water-based surface treatment composition containing urethane resin and/or acrylic resin, curing agent, silica powder, oxidized polyethylene wax of specified particle size, and a compound with a special structure as dispersing agent.
This invention also relates to surface-treated metal sheet that characteristically comprises metal whose surface carries a first layer comprising a chromate coating layer having a coating weight that is 3 to 100 milligrams per square meter (hereinafter usually abbreviated as xe2x80x9cmg/m2xe2x80x9d) as chromium metal and a second layer with a coating weight from 0.3 to 3.0 grams per square meter (hereinafter usually abbreviated as xe2x80x9cg/m2xe2x80x9d) of a resin coating layer that has been formed by the application of the above-described water-based surface treatment composition followed by drying.
A water-based treatment composition according to the invention comprises, preferably consists essentially of, or more preferably consists of, water and the following components:
(A) dissolved, dispersed, or both dissolved and dispersed urethane resin, acrylic resin; or both urethane and acrylic resins;
(B) dissolved, dispersed, or both dissolved and dispersed curing agent molecules;
(C) dispersed silica powder;
(D) dispersed oxidized polyethylene wax with an average particle size of 0.01 to 0.2 xcexcm; and
(E) dissolved, dispersed, or both dissolved and dispersed molecules that conform to the immediately following general chemical formula (I): 
xe2x80x83wherein: R1 represents a C1 to C20 alkyl moiety or a C2 to C20 alkenyl moiety; R2 represents a block homo-oligomer of oxyethylene or a block co-oligomer of oxyethylene and oxypropylene that conforms to the general chemical formula xe2x80x94(C2H4O)mxe2x80x94(C3H6)nxe2x80x94, wherein m represents an integer from 5 to 20 and n represents an integer from 0 to 10; R3 represents a hydrogen moiety or SO3M, where M represents a hydrogen atom, an alkali metal ion, or an ammonium ion; and R4 represents a hydrogen atom, a C1 to C4 alkyl moiety, or a C2 to C4 alkenyl moiety,
wherein: the total solids from components (A) and (B) constitute from 50 to 95% by weight of the total solids from components (A) through (E); the solids from component (C) constitute from 3 to 40% by weight of the total solids from components (A) through (E); the total solids from components (D) and (E) constitute from 2 to 20% by weight of the total solids from components (A) through (E); and the ratio by weight of solids from component (A) to solids from component (B) is from 4:1.0 to 49:1.00.
The solids from component (E) preferably make up from 10 to 40% by weight of the total solids from components (D) and (E). Independently, the glass-transition temperature of component (A) is preferably from xe2x88x9240 to 0xc2x0 C. Also independently of both other preferences in this paragraph, component (B) is preferably selected from epoxy resins and more preferably from epoxy resins that contain at least three epoxy groups in each molecule.
A urethane resin to be used in component (A) of a composition according to this invention preferably is synthesized from four types of starting materials: polyol molecules, polyisocyanate molecules, carboxylic acid molecules, and molecules that contain at least three active hydrogens.
The polyol molecules are exemplified by: ethylene oxide and/or propylene oxide adducts of low-molecular-weight polyols such as ethylene glycol, diethylene glycol, triethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,2-butylene glycol, 1,3-butylene glycol, 1,4-butylene glycol, hexamethylene glycol, hydrogenated bisphenol A, bisphenol A, trimethylolpropane, and glycerol; polyether polyols such as polyethylene glycols, polypropylene glycols, polyethylene/polypropylene glycols, polycaprolactone polyols, polyolefin polyols, and polybutadiene polyols; and hydroxyl-terminated polyester polyols afforded by the reaction of the aforesaid polyols and a polybasic acid such as succinic acid, glutamic acid, adipic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid, tetrahydrophthalic acid, endomethylenetetrahydrophthalic acid, and hexahydrophthalic acid.
The polyisocyanate molecules can be exemplified by aliphatic, alicyclic, and aromatic polyisocyanates. Preferred examples of the polyisocyanate component are tetramethylene diisocyanate, hexamethylene diisocyanate, lysine diisocyanate, hydrogenated xylylene diisocyanate, 1,4-cyclohexylene diisocyanate, 4,4xe2x80x2-dicyclohexylmethane diisocyanate, 2,4xe2x80x2-dicyclohexylmethane diisocyanate, isophorone diisocyanate, 3,3xe2x80x2-dimethoxy-4,4xe2x80x2-biphenylenediisocyanate, 1,5-naphthalenediisocyanate, 1,5-tetrahydronaphthalene diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, 4,4xe2x80x2-diphenylmethane diisocyanate, 2,4xe2x80x2-diphenylmethane diisocyanate, phenylene diisocyanate, xylylene diisocyanate, and tetramethylxylylene diisocyanate. Among these, coatings with a particularly good corrosion resistance and chemical resistance are obtained by the use of aliphatic and alicyclic polyisocyanates, e.g., tetramethylene diisocyanate, hexamethylene diisocyanate, lysine diisocyanate, hydrogenated xylylene diisocyanate, 1,4-cyclohexylene diisocyanate, 4,4xe2x80x2-dicyclohexylmethane diisocyanate, 2,4xe2x80x2-dicyclohexylmethane diisocyanate, and isophorone diisocyanate.
The carboxylic acid molecules can be exemplified by 2,2-dimethylolpropionic acid, 2,2-dimethylolbutyric acid, and 2,2-dimethylolvaleric acid.
The molecules containing at least three active hydrogens can be exemplified by melamine, diethylenetriamine, trimethylolpropane, pentaerythritol, glycerol, and their ethylene oxide and/or propylene oxide adducts.
A compound containing two active hydrogens (chain extender) as typically used in urethane resin synthesis can also be used as necessary or desired for synthesis of the urethane resin under consideration. These chain extenders can be exemplified by polyols such as ethylene glycol, propylene glycol, neopentyl glycol, 1,6-hexanediol, and the ethylene oxide and/or propylene oxide adducts of the preceding, and by amines such as ethylenediamine, propylenediamine, hexamethylenediamine, tolylenediamine, xylylenediamine, diaminodiphenylmethane, diaminodicyclohexylmethane, piperazine, 2-methylpiperazine, isophoronediamine, succinic dihydrazide, adipic dihydrazide, and phthalic dihydrazide.
The starting components for the urethane resin used in this invention are preferably employed in the following quantities, in each case per 100 weight parts of the urethane resin: the polyol component at 30 to 70 weight parts and preferably 35 to 65 weight parts, the polyisocyanate component at 20 to 50 weight parts and preferably 25 to 40 weight parts, the carboxylic acid component at 0.5 to 10 weight parts and preferably 1 to 8 weight parts, and the compound containing at least three active hydrogens at 0.1 to 5 weight parts and preferably 0.2 to 3 weight parts. The chain extender, when used, preferably is used at 1 to 15 weight parts and preferably 3 to 10 weight parts, in each case per 100 weight parts urethane resin.
The procedure for synthesizing the urethane resin used in this invention is not critical and those procedures known in the concerned art can be used; however, synthesis by the industrially widely used prepolymer technique is preferred. In the prepolymer technique, the urethane resin is synthesized by (i) reacting the polyol component, polyisocyanate component, carboxylic acid component, and compound containing at least three active hydrogens in an organic solvent that is inert to the reaction and that has a high affinity for water and (ii) dispersing the resulting polymer in an aqueous solution containing neutralizing agent and the chain extender.
The aforesaid organic solvent that is inert to the polymerization reaction is exemplified by acetone, methyl ethyl ketone, dioxane, tetrahydrofuran, N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, dimethyl sulfoxide, and ethyl acetate. The neutralizing agent can be exemplified by organic amines such as trimethylamine, triethylamine, tripropylamine, tributylamine, N-methyldiethanolamine, and triethanolamine, and by inorganic bases such as sodium hydroxide, potassium hydroxide, and ammonia.
The molecular weight of the urethane resin product is not critical, but molecular weights of at least 5,000 are preferred.
Acrylic resins suitable for use in this invention as part or all of component (A) are copolymers afforded by the copolymerization of acrylic and/or methacrylic acid with another monomer containing an ethylenic double bond. This other monomer is exemplified by maleic acid, maleic anhydride, fumaric acid, crotonic acid, itaconic acid, citraconic acid, cinnamic acid, 2-hydroxyethyl (meth)acrylate1, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 3-hydroxybutyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 2-hydroxyethyl (meth)allyl ether, 3-hydroxypropyl (meth)allyl ether, 4-hydroxybutyl (meth)allyl ether, allyl alcohol, glycidyl (meth)acrylate, 2-(1-aziridinyl)ethyl acrylate, vinyltrimethoxysilane, vinyltriethoxysilane, allyl glycidyl ether, iminol methacrylate, acryloylmorpholine, N-methylol(meth)acrylamide, N-methoxymethyl-(meth)acrylamide, N-ethoxymethyl(meth)acrylamide, N-butoxymethyl(meth)acrylamide, (meth)acrylamide, N-methyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide, N-dimethylaminopropyl(meth)acrylamide, vinyl formate, vinyl acetate, vinyl butyrate, vinyl acrylate, styrene, xcex1-methylstyrene, tert-butylstyrene, vinyltoluene, (meth)acrylonitrile, cinnamonitrile, (meth)acryloxyethyl phosphate, bis(meth)acryloxyethyl phosphate, and (meth)acryloxyethyl phenyl acid phosphate.
1The symbol xe2x80x9c(meth)xe2x80x9d in any following chemical name means xe2x80x9cmethyl substituted or unsubstitutedxe2x80x9d. 
The (meth)acrylic acid is used preferably at from 1 to 20 weight parts and more preferably at from 2 to 15 weight parts, in each case per 100 weight parts of the acrylic resin used in this invention.
The procedure for synthesizing the acrylic resin used by this invention is not critical, but a procedure that has proven satisfactory on an industrial basis is preferably employed. An example of one such procedure is emulsion polymerization, in which polymerization is carried out after the monomer has been homogeneously dispersed and emulsified in water using a surfactant known as an emulsifying agent. An example of another such procedure is suspension polymerization, in which the monomer is dispersed in a solventxe2x80x94such as waterxe2x80x94in which the monomer is either completely or almost completely insoluble and the polymerization reaction is then carried out in the small drops of suspended monomer using a polymerization initiator that is soluble in the monomer but poorly soluble in the solvent.
While the molecular weight of the acrylic resin used in this invention is not critical, molecular weights of 10,000 to 1,000,000 as measured by gel permeation chromatography (hereinafter usually abbreviated as xe2x80x9cGPCxe2x80x9d) are preferred.
The resin (A) used in this invention preferably has a glass-transition temperature from xe2x88x9240 to 0xc2x0 C. and more preferably from xe2x88x9240 to xe2x88x925xc2x0 C. Glass-transition temperatures below xe2x88x9240xc2x0 C. are undesirable because they produce a poor blocking resistance in the ultimately obtained coating. Moreover, the physical properties of the resin coating undergo a major change across the glass-transition temperature boundary. Since after the molding operation the moldings will generally be handled at temperatures in the range from 10 to 50xc2x0 C., a glass-transition temperature in this range is very likely to result in changes in the physical properties of the resin coating merely as a consequence of changes in air temperature and therefore in a corresponding deterioration in the coating""s resistance to damage. In addition, glass-transition temperatures in excess of 50xc2x0 C. result in a deterioration in the film-forming properties of the coating, which necessitates an uneconomical raising of the temperature at which the treatment composition is dried. In sum, then, the glass-transition temperature is preferably in the range from xe2x88x9240 to 0xc2x0 C.
The glass-transition temperature of the resins was determined from the inflection point in the rate of elasticity loss in measurement at a frequency of 100 Hertz using a Rheograph Solid S-1 instrument (from Kabushiki Kaisha Toyo Seiki Seisakusho). The measurement specimen was a film with a thickness of 100 xcexcm, width of 8 millimeters (hereinafter usually abbreviated as xe2x80x9cmmxe2x80x9d), and length of 30 mm. The specimen was dried for 30 minutes at 100xc2x0 C. prior to measurement.
A curing agent (B) is used by this invention in order to bring about a more prominent manifestation of the properties of the resin (A). The curing agent (B) used in this invention is preferably an isocyanate compound, aziridine compound, or epoxy resin. An epoxy resin containing at least three epoxy groups in each molecule is most preferred for use as the curing agent (B). The resin used in this invention as component (A) contains carboxylic groups, and major enhancements in coating properties are obtained by the formation of a three-dimensional network structure in the produced resin through the reaction of these carboxyl groups and the functional groups (isocyanate group, aziridinyl group, or epoxy group) in the curing agent. The epoxy resin under consideration can be exemplified by novolac epoxy resins and the epoxy resins afforded by the reaction of epichlorohydrin with a compound containing at least three OH groups. The molecular weight of the epoxy resin used in this invention is not critical, but molecular weights no greater than 3,000 as measured by GPC are preferred.
The solids weight ratio between components (A) and (B) is more preferably from 10:1.0 to 40:1.00. An (A):(B) ratio below 4:1.0 does not usually result in a satisfactory manifestation of the properties of the resin (A). Moreover, unreacted curing agent will usually remain at an (A):(B) ratio below 4:1.0, and any residual unreacted curing agent will function as a plasticizer and thereby reduce the corrosion resistance. The effects from the addition of curing agent are so weak at an (A):(B) ratio in excess of 49:1.00 that a satisfactory corrosion resistance usually cannot be obtained.
The total solids from components (A) and (B) must make up 50 to 95% by weightxe2x80x94and preferably make up 60 to 85% by weightxe2x80x94of the grand total solids from all the components (A), (B), (C), (D), and (E) in the water-based surface treatment composition of this invention. The water resistance of the coating itself is reduced when this value falls below 50% by weight. The corrosion resistance in damaged areas is reduced when this value exceeds 95% by weight.
The particle size, particle morphology, and type are not critical for the silica powder component (C) used in this invention, but the range of 3 to 30 nanometers (hereinafter usually abbreviated as xe2x80x9cnmxe2x80x9d) is preferred for the particle size. The silica powder will occur dispersed in the water in the water-based surface treatment composition of this invention. The silica powder must make up 3 to 40% by weightxe2x80x94and preferably makes up 10 to 30% by weightxe2x80x94of the total solids. A proportion below 3% by weight usually results in little improvement in the corrosion resistance, while a proportion in excess of 40% by weight usually results in a weak binder activity by the resin component (A) and hence in a reduced corrosion resistance.
The oxidized polyethylene wax component (D) used in this invention is added in order to improve the resistance to damage. The average particle size of the oxidized polyethylene wax (D) must be from 0.01 to 0.2 xcexcm and is preferably from 0.05 to 0.18 xcexcm. As a general rule, the friction coefficient, which is used as an index of lubricity, declines as the wax particle size increases. However, the friction coefficient does not necessarily strictly correlate with resistance to damage, and the relationship (scuffing resistance) between the number of sliding traverses (or sliding distance) and the friction coefficient is more important. As a result of extensive investigations into the relationship between the average wax particle size and the number of sliding traverses, the inventors have discovered that an excellent resistance to damage is obtained at an average wax particle size of 0.01 to 0.2 xcexcm. Since the average wax particle size depends on the melt viscosity of the wax and the performance of the apparatus used for dispersion, average wax particle sizes below 0.01 xcexcm are uneconomical due to their requirement for higher performance equipment. In the case of sizes in excess of 0.2 xcexcm, the wax protruding from the coating surface is easily removed during sliding and the continuous sliding properties are impaired as a result.
The molecular weight and melting point of the oxidized polyethylene wax (D) are not critical to this invention, but this component preferably has an acid value in the range from 5 to 50 and more preferably in the range from 10 to 30. An acid value below 5 results in an almost complete absence of miscibility between the wax and resin, which results in an almost complete segregation of the wax at the coating surface during coating formation with a concomitant reduction in damage resistance and paint adherence. Due to the strong hydrophilicity of the wax at an acid value in excess of 50, the wax itself has a reduced lubricity and the damage resistance is thus impaired.
The dispersing agent (E) used in this invention is a compound conforming to general chemical formula (I) as given above. R1 in formula (I) is preferably a C5 to C20 alkyl moiety or a C2 to C5 alkenyl moiety; m is preferably an integer from 8 to 20; n is preferably an integer from 0 to 5; R3 is preferably a hydrogen atom or S03NH4; and R4 is preferably a hydrogen atom or a C2 to C4 alkenyl moiety. The oxidized polyethylene wax component (D) employed in this invention will generally be used as the water-based dispersion produced using the dispersing agent component (E). The method for dispersing the oxidized polyethylene wax is not critical and those methods used industrially can be used here. The average particle size of the oxidized polyethylene wax (D) referenced above is the value occurring in the aforesaid water-based dispersion of this component.
The solids from component (E) preferably make up from 5 to 40% by weight and more preferably from 10 to 30% by weight of the total solids from components (D) and (E). The dispersion stability of the oxidized polyethylene wax usually will be unsatisfactory when this value is below 5% by weight, while the water resistance of the coating obtained using the corresponding water-based surface treatment composition will be impaired when this value exceeds 40% by weight.
With respect to the proportion in which this water-based dispersion is used, the total solids from components (D) and (E) must make up from 2 to 20% by weight, preferably make up from 3 to 15% by weight, and more preferably make up from 3 to 10% by weight, of the grand total of solids in the water-based surface treatment composition of this invention from all components (A) through (E). Little improvement in damage resistance is usually obtained at less than 2% by weight, while the topcoat paintability is often impaired above 20% by weight.
In regards to optional components, the water-based surface treatment composition of this invention may contain a surfactant known as a wetting improver in order to promote the formation of a uniform coating on the substrate, an electrically conductive substance in order to enhance the weldability, colored pigment to enhance the aesthetics, and solvent in order to improve the coating-forming performance.
The water-based surface treatment composition of this invention can be prepared by mixing the above-described components (A), (B), (C), and a separate water-based dispersion made by dispersing component (D) in a solution of component (E) in water, along with any of the above-described optional components if desired. The order of component addition otherwise is not critical, and mixing can be effected, for example, by stirring with a propeller-type stirrer.
The resulting water-based surface treatment composition of this invention preferably has a solids concentration in the range from 5 to 50% by weight and more preferably from 5 to 40% by weight. A solids concentration below 5% by weight should be avoided for the correspondingly long drying times it entails, while a solids concentration in excess of 50% by weight causes the treatment composition itself to have a high viscosity and hence runs the risk of causing handling problems.
Metals that can be coated with the water-based surface treatment composition of this invention can be exemplified by metal sheet such as cold-rolled steel sheet, zinciferous-plated steel sheet, aluminiferous-plated steel sheet, and aluminum sheet.
The water-based surface treatment composition of this invention can be directly painted on the above-described metals. However, in order to fully manifest the advantageous effects of this invention, a chromate coating is preferably first produced by chromate treatment by the usual methods and the water-based surface treatment composition of this invention is then coated thereon in order to form a resin coating.
Chromate treatments are exemplified by electrolytic chromate treatments in which the chromate coating is produced by electrolysis; reactive chromate treatments in which the coating is formed by reaction with the substrate and the excess treatment bath is subsequently washed off; and dry-in-place chromate treatments in which the treatment bath is applied to the workpiece and a coating is produced by drying without a water rinse. Any of these chromate treatment methods can be used with this invention.
The formation of a resin coating can be effected by application of the water-based surface treatment composition of this invention at 10 to 50xc2x0 C. on the undercoating formed by chromate treatment as described above and thereafter drying under conditions such that the highest temperature reached by the underlying metal during drying was 60 to 200xc2x0 C. The water-based surface treatment composition can be applied by the usual methods, for example, roll coating, immersion, or electrostatic coating.
The surface-treated metal sheet of this invention that is produced by the herein-above-described two-step treatment characteristically comprises metal whose surface carries a first layer comprising a chromate coating layer having a weight of 3 to 100 mg/m2 as chromium metal and a second layer comprising 0.3 to 3.0 g/m2 of a resin coating layer as formed by the application and drying of the water-based treatment composition for metal surfaces of this invention. The weight of the chromate coating layer is preferably from 3 to 50 mg/m2 as chromium metal and more preferably is from 5 to 40 mg/m2 as chromium metal.
Little improvement in corrosion resistance is obtained at a chromium add-on below 3 mg/m2, while add-ons in excess of 100 mg/m2 are uneconomical because no additional improvement in corrosion resistance is obtained at such levels. The improvement in corrosion resistance is usually inadequate at a resin coating weight (dry basis) below 0.3 g/m2, while weights in excess of 3.0 g/m2 are uneconomical again because no additional improvement in corrosion resistance is obtained at such levels.
The resin coating formed by the application and drying of the water-based surface treatment composition of this invention and hence the corresponding surface-treated metal sheet exhibit an excellent corrosion resistance in flat areas, an excellent corrosion resistance in damaged areas, and an excellent paint adherence. No particular restrictions apply to the topcoat that may be painted on the resin coating, and the topcoat is exemplified by ambient temperature-drying melamine-alkyd paints, bakable melamine-alkyd paints, acrylic resin paints, and ultraviolet light-curing resin paints.